Porous subsurface sedimentary rock formations are typically saturated with fluids. The fluids may be entirely water, however, in some subsurface formations, the water in the pore spaces may have been displaced by hydrocarbons such as oil and gas. Thus, some present day subsurface formations may have pore spaces containing water, gas or oil, or mixtures thereof.
Detection of formations with oil or gas present in the pore spaces is of significant economic interest. Certain techniques for detection of such formations include determining the existence of subsurface electrical resistivities that are anomalously high. The principle of such detection is based on the fact that the flow of electric current through a porous rock formation is related to the fractional volume of the pore spaces with respect to the total rock volume, the spatial configuration of the pore spaces, and the electrical properties of the fluids filling the pore spaces. For example, brine-saturated porous rock formations are typically much less resistive than the same rock formations having hydrocarbons in some or all of the pore spaces because brine is a relatively good electrical conductor while hydrocarbons are typically good electrical insulators.
Various techniques for measuring the electrical resistivity of subsurface rock formations are known in the art and may include imparting an electromagnetic field into the subsurface formations and measuring electromagnetic fields induced in the subsurface formation in response to the imparted electromagnetic field. For such measurement techniques, the electromagnetic field may be imparted using an electromagnetic field transmitter, for example, by passing an electric current through a dipole electrode pair.
The electric current used to impart the electromagnetic field may be controlled to provide a transient electromagnetic field, and the responses measured by electromagnetic receivers may be related to transient response of the formations in the earth's subsurface. Transient electromagnetic fields may be imparted by switching the transmitter current on, switching the transmitter current off, reversing the polarity of the transmitter current, or combinations thereof. The electromagnetic field induced by the transmitter typically has a very small voltage and may have poor signal quality due to electrical noise. It is desirable to mitigate or reduce electrical noise during low voltage measurement.